1. Field of the Invention
The present invention relates to an intermediate gradient image forming method whereby gradients are expressed using dots of a coloring agent.
2. Description of the Prior Art
Hitherto, as a method for such a kind of gradient expression, in general, there is a density modulating method such as a dither method, a density patterning method or the like whereby densities of constant dots are controlled in accordance with light and shade of an image.
However, in case of obtaining smoother gradients by these methods, there are problems such that the size of a dither matrix has to be set to be large and in the density patterning method, the number of submatrices which are derived by dividing the dither matrix has to be made large, so that the effective pixel size is enlarged and resolution deteriorates.
For such problems, for example, as set forth in the Japanese Patent Unexamined Publication No. 60878/1983, there has been proposed a method whereby the number of kinds of dots which are used is set to a plural number by making dot diameters, ink concentrations or the like different and what is called a multi-value conversion is performed, thereby deriving a picture quality having excellent gradient and resolution.
An explanation will be made hereinbelow with respect to an example of an ink jet printer.
FIG. 1 shows an example whereby each of the dot diameter and ink concentration is made different to obtain two different kinds of values, and the multivalue conversion of total five values consisting of printing (1) no dot, (2) a small dot of low-concentration ink, (3) a large dot of low-concentration ink, (4) a small dot of high-concentration ink, and (5) a large dot of high-concentration ink, and then these five values are density modulated by way of a matrix of 2.times.2.
In this way, a total of seventeen gradients can be expressed. However, in this case, the concentrations of the inks which are used in the eighth and twelfth gradients differ. Namely, the ink concentration is switched from the low-concentration ink to the high-concentration ink on a four-stage unit basis. Therefore, when the density suddenly changes, a pseudo outline is generated in that change portion due to the difference in ink concentration, so that picture quality deteriorates.